Harvesting crops manually is a labor-intensive activity, in particular, the application of pesticides/fertilizers. Due to public concern in regards to the release of chemicals into the environment and human exposure to chemicals, the unmanned aerial vehicles (UAVs) may be utilized in aerial application of spray material (e.g., spraying pesticide/fertilizer). Employing operators to remotely control UAVs, however, is labor-intensive and requires long periods of high-concentration to attention.
For UAVs aiming to deliver pesticides/fertilizers directly to crops, an important requirement is that they must be able to fly at low constant heights above the target crops to ensure efficiency, minimal waste, as well as complete and even coverage. Flying too high above the crop results in large amounts of pesticides/fertilizers being lost into the open air. This not only poses a health and environmental hazard, but also fails to deliver the necessary pesticides/fertilizers to the crops. On the other hand, flying too close to the crops limits the coverage of the spray, increases the danger of colliding with crops, and poses a risk to the UAVs should the terrain rise. Thus arises the need for maintaining the UAV at low fixed heights above the crops. In other words, it is desirable for the UAVs to be able to adapt to and follow changing terrain, whether it be elevation or obstacles, while maintaining a relatively constant height above the crops/ground. Such capability is important for UAVs to accurately deliver pesticides/fertilizers to crops planted in areas such as valleys, hills, and terraces.